14250 battery

Revealing the core technology of 14250 battery

As one of the core components of pure electric vehicles, power batteries are closely related to the vehicle's cruising range, curb weight, power performance, and handling performance. In terms of the manufacturing cost of pure electric vehicles, batteries account for the highest proportion, generally more than 30%, which leads to higher prices and later maintenance costs for electric vehicles. Therefore, reducing the unit cost of batteries and increasing the energy density of batteries have always been the main direction of the development of electric vehicle technology. For BYD, which originally started with batteries, high-performance batteries are one of BYD's killer features. Especially after replacing the ternary lithium battery with higher energy density, higher discharge voltage, and better low-temperature performance, the core competitiveness of BYD's EV model series has been greatly improved. In this issue, we will fully disassemble the battery pack of BYD Qin ProEV500 and analyze BYD's innovations and management technologies such as battery pack safety design and thermal management design. Square aluminum shell integration process After uncovering the ultra-thin non-metallic cover of the battery pack and the silica aerogel fireproof and heat-insulating layer, we can clearly see the overall layout structure of the battery pack, of which the most intuitive is the integration process of the battery pack. Integration technology is very important in the development of power batteries. It must meet all safety requirements such as mechanical protection, thermal safety protection, thermal management, and environmental protection, while pursuing lightweight and cost optimization. Unlike the cylindrical battery cell method adopted by Tesla, BYD uses a square aluminum shell with a higher domestic popularity rate, which has the advantages of high energy density and low integration difficulty. In addition, the square packaging process also helps to reduce the gap between the batteries, making the overall size more compact, while the cylindrical battery cells must leave triangular gaps between the batteries, reducing the space utilization rate. The battery cell shell made of magnesium-aluminum alloy is lighter and cheaper than the stainless steel shell used in cylindrical batteries, which is conducive to improving the energy density of the battery cell and has lower manufacturing costs. Moreover, the structure of the square shell can accommodate more electrolyte, the battery cell pole piece has lower expansion stress, and the battery life is more than twice longer than the cylindrical battery module. The Qin ProEV500 uses BYD's independently developed nickel-cobalt-manganese ternary battery, that is, based on lithium cobalt oxide, it has been improved to use nickel-cobalt-manganese as the positive electrode material of the battery, and the proportion of nickel-cobalt-manganese is reasonably matched. While optimizing costs and ensuring safety, the battery has excellent electrochemical properties such as high capacity, good thermal stability, and wide charging and discharging voltage. And effectively improve the battery energy density to 160.9Wh/kg, combined with a capacity of 56.4kWh. Achieve a NEDC range of 420km and a constant speed range of 500km at 60km/h, which effectively alleviates users' concerns about the range. And thanks to the high energy density of the battery pack, the battery load of the car is effectively reduced, thereby reducing the weight of the car. The grouping method of the battery module fully takes into account the needs of heat dissipation and lightweight. The two sides of the aluminum short plate are tied with elastic steel belts to adapt to the expansion of the battery during charging and discharging. At the same time, modules of various specifications can achieve flexible layout to meet the needs of different models. The middle part of the car body is as flat as possible, with a single-layer layout to increase the height space in the car. In terms of detailed design, aluminum bars are used for the main circuit connection and its signal acquisition part. Under the same conductivity, the weight can be reduced by more than half compared with copper materials, and the cost can also be controlled. However, we found that copper bars were used instead of aluminum bars on the lead-out poles. This is because the hardness of aluminum bars is low. Under high temperature and high stress, aluminum will collapse, and it is not easy to rebound after collapse. The hot and cold will cause the gap to increase, the contact resistance to increase, and bring safety hazards. In the connection of different materials of copper and aluminum, BYD uses a technology called electromagnetic pulse welding. Compared with the commonly used direct rolling connection or ultrasonic welding technology of copper and aluminum, the process of electromagnetic pulse welding is more difficult. Although the cost will also increase accordingly, the effect is the best and it is currently a more advanced technology. Between each battery pole and pole, lasers are also used to weld the aluminum busbar and pole together to ensure reliability. And there is a depression designed on the busbar to absorb mechanical vibration and stress caused by electric shock expansion. If it is a straight aluminum bar, as the battery ages and expands, the distance between the poles of adjacent batteries will increase, and the tensile stress will affect the reliability of the solder joints. In the signal connection part, BYD uses a flexible circuit board, which is more integrated and thinner than the traditional sampling harness solution. If you look closely, you will find that there are filamentary wiring on the flexible circuit board, which we call the sampling line fuse. Its function is that in the event of a collision, it may squeeze the sampling harness and cause a short circuit, which in turn causes the sampling line to catch fire. These filaments will melt due to overcurrent during a short circuit, thereby cutting off the short circuit circuit and ensuring the safety of the entire harness and the battery module. Since the battery management system uses lithium batteries, in order to ensure that the battery is always working within a relatively suitable temperature range, BYD has equipped it with an independent battery intelligent temperature control management system to ensure that the power battery can obtain stable and reliable performance under complex temperature environments. This intelligent temperature control management system can effectively ensure the uniformity of battery temperature through liquid medium insulation and cooling. In terms of cooling methods, BYD has added a heat dissipation circuit in the battery, which is connected to the air conditioning circuit through a plate heat exchanger. Temperature sensors are installed at the battery water inlet and outlet and the battery level ears. The power of the air conditioning compressor is adjusted in real time in combination with the battery temperature to control the battery water inlet temperature and flow rate, so as to control the battery temperature at a suitable working temperature. In terms of heating methods, BYD connects a PTC water heater in series in the battery heat dissipation circuit, and controls the water inlet temperature and flow rate by adjusting the power of the water heater, so as to control the battery to work at a suitable temperature in winter and ensure charging speed and discharge power. And through the battery management system BMS, the battery status is monitored in real time, and protection is provided for low temperature, overcharge, over-discharge, overtemperature, etc., thereby extending the battery life. When the temperature is too low or too high, the charging and discharging power will be limited, and when the temperature is seriously too low or too high, charging and discharging will be prohibited to protect the battery. The serpentine water-cooling flat tubes are used for cooling and heating of water pipes arranged at the bottom or side of different battery modules. At the same time, we noticed that the water pipes in the battery pack use the same harmonica pipes as Tesla. This harmonica pipe is very thin, with a wall thickness of 0.8-1mm, which is much lighter than the traditional aluminum alloy water pipe with a wall thickness of 1.6-2mm. What is more distinctive is that the transverse bending serpentine design used on the Qin ProEV500 can be said to use the same technical route as Tesla, but it is more difficult from a process point of view, especially in the outer circle of the curved part. The elongation rate of the inside and outside of the material is quite different, and wrinkles and cracks are prone to occur, which requires very high materials and processes. The benefits of doing so are also obvious. Tesla's pipeline is to "wrap" the battery from the side, but the problem is that the contact surface between the cylindrical battery and the heat dissipation pipeline is almost a straight line, which is inefficient. This is why the latest 21700 (Model 3) battery module adopts an overall glue filling method, which can only sacrifice "weight" for "heat". BYD's pipeline design works well with square batteries. The pipeline is completely attached to the side wall of the battery to maximize the contact area. This design not only ensures that each battery cell can be cooled, but also achieves a very good lightweight effect compared to the cooling water channel designed with a whole piece of aluminum plate. This is a leading technology in the entire industry and has completed a challenge for BYD. Assembly process The entire battery pack is perfectly controlled during the assembly process. In particular, there are basically two or three confirmations on each water cooling pipe connection point, each connector connection point, each high-voltage electrical connection point, and the structural fixed point. For example, some low-voltage connectors are responsible for battery signal acquisition. If the BMS system loses the single cell voltage signal or the single cell temperature signal, it cannot continue to work reliably, and the battery safety cannot be fully guaranteed. The general connector has only one lock, and there will be a locking sound as a prompt after locking. BYD not only has a sound as confirmation, but also a secondary lock. Only when the primary lock is plugged in place can the secondary lock be closed. The two-stage locking design is in place. In addition, the connection of high-voltage electrical appliances is also the core and most critical point in the entire battery pack assembly, especially in the reliability and low internal resistance design of the main circuit connection. BYD's battery pack uses high-temperature resistant polyimide pressure-sealed copper bars for long-distance connections in the main circuit, and designs many three-dimensional bends, so that when it is vibrated or expanded by heat, these bends can absorb the change in length and avoid transferring the load to the connecting screws. Although from the perspective of contact internal resistance, the contact internal resistance of a single screw meets the heating requirements. But BYD still insists on using a double screw design, which greatly improves reliability. And in the confirmation of the tightening of the screws, we found three colors of color codes, which means that three confirmations were made. The first time is to tighten the automatic tightening shaft and mark it with a red mark, and the next two times are manual re-inspections using a torque wrench, marked with yellow and white marks respectively. In addition, most of the pipelines in the entire battery pack are made of nylon mesh braided sleeves, especially the pipelines that are in contact with the battery pack shell and internal devices, which not only protect the wiring harness and avoid wear, but also reduce noise. In summary, BYD Qin ProEV500 has made a lot of efforts in the lightweight and reliability of the entire battery pack, and has improved the energy density of the battery by improving the battery cell ratio, optimizing the battery management system and active thermal management technology, thereby improving the power, handling and endurance performance of the vehicle. Especially in the design of safety, BYD's engineers have considered it more carefully, so as to protect the user's driving safety to the greatest extent. All of the above reflect BYD's technical advantages and development space in the field of battery research and development, and it can be said that it has led the direction of technology development in the industry.

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